1. Field of the Invention
The invention pertains to a ground compaction device.
2. Description of the Related Art
These types of vibration plates commonly consist of an upper mass that has a drive unit and a lower mass that consists essentially of a ground contact plate and an oscillator unit. In this oscillator unit there are commonly one, two, or four imbalance exciters that are arranged on one or two exciter shafts driven by the drive unit. The upper mass is coupled at a number of places to the lower mass through spring-damper units, e.g. rubber bumpers. By rotating the exciter shafts, forces are produced that lift the ground contact plate in intervals andxe2x80x94if the exciter is configured correctlyxe2x80x94shift it a bit in the direction desired by the user. In this way, the vibration plate not only can be moved forward and backward, but also can be steered.
Small vibration plates are steered by the user using a center pole, which can be done by the user applying force as required; this is due to the relatively low weight of the vibration plate. For larger vibration plates, on the other hand, the steering is done using the oscillator. In this arrangement, imbalance forces are distributed along the axial direction of the exciter shaft, i.e. on the right and left halves of the lower mass. By activating a switch at the center pole, a hydraulic mechanism is activated that influences the position of the centrifugal weights relative to one another such that asymmetrical force vectors act on the ground contact plate, resulting in a yaw moment being produced about the vertical axis of the machine, which then steers the machine.
Especially for larger vibration plates, but for smaller plates as well, the steering ability either requires a lot of force on the part of the user or is only mechanically possible by using expensive hydraulics.
From DE-B-1 168 350, a ground compaction device is known that has a lower mass containing a ground contact plate and an oscillator unit, and that has spring units that couple the upper mass to the lower mass at a number of places. The spring tension of the spring units is changed using piston-cylinder units at the upper mass associated with each. This allows the ground pressure force of the ground contact plate to be adjusted. The upper mass is essentially made up of a steamroller.
EP-A-0 142 198 shows a comparable ground compaction device in which the spring tension on the spring units can also be shifted by a piston-cylinder unit to change the surface pressure of the ground contact plate.
From U.S. Pat No. 3,802,791, a ground compaction device is known that has an upper mass containing an oscillator unit and a lower mass having a ground contact plate. The oscillations produced at the upper mass are transferred through an intermediate plate and through spring units to the lower mass. Between the intermediate plate belonging to the upper mass and the lower mass are additional spring-damper units installed at a number of places, whose spring tension can be changed by tilting the upper mass in the forward or backward direction of travel. Steering the ground compaction device is done by merely pulling on a guide bracket.
The objective of the invention is to provide a vibration plate with a directional control system that is a simple design but also very effective.
It has been shown that by changing the damping characteristics or parameters of at least one damping element, which can be located on the right or left side of the vibration plate, asymmetrically with respect to the direction of travel, a yaw moment about the vertical axis of the plate is produced so that the plate turns in a direction accordingly. Changing the spring or damping characteristics using the steering device can thus turn the vibration plate into any desired direction accordingly during operation by the user.
In a preferred embodiment form of the invention, at least one spring-damper unit has a damping material made of viscoelectric liquid that can be subjected to a variable electric voltage by the steering unit. When using a viscoelectric liquid in a damper, it is possible to vary the damping characteristics from completely non-damping to rigid and thus adjust the damping value as desired, depending on the voltage applied.
Preferably, one or more of these types of damper are used together with rubber bumpers that couple the upper mass to the lower mass in a known fashion. In this regard, the viscoelectric dampers are arranged parallel to the rubber bumpers.
In another advantageous embodiment form, the damping characteristics of a selected rubber bumper is changed by providing a rod that can be moved into or out of the rubber bumper by means of a servomechanism controlled by the steering unit.
Again, another embodiment form of the invention is characterized in that the spring-damper unit whose spring and/or damping characteristics can be changed, has a number of springs and/or dampers that can be individually turned on or off by the steering unit by means of a servomechanism. This makes it possible, for example, to vary the position at which the rubber bumpers are coupled to the lower or upper mass by interposing springs or other rubber bumpers. Likewise, it is also possible to completely bypass individual rubber bumpers using a magnetic latch, or to switch in another rubber bumper.